(((Alison))), I'm so sorry your kids are going through this, especially D17.
FWIW, don't you think the attachment to S15 is because your H is operating at about that level right now---teenage boy? And if he was really attached to D17 before, she probably brings out the guilt, whether she means to or not. And we know how MLCers handle guilt...
My XH is doing a little of this, treating S15 as a sometimes buddy/confidant where he wouldn't have before. I can tell sometimes it makes S15 uncomfortable to know things that I don't, and I don't like it that he has to feel that way either.
JMO, of course. But I do think Tia has a point, your kids are old enough to understand MLC, and it may help them to feel better about themselves in all this mess.